1. Field of the Invention
The invention generally relates to estimating error in a WLAN-based positioning system and, more specifically, to determining the expected error of an estimated position of a WLAN-enabled mobile device using WLAN-based positioning system.
2. Discussion of Related Art
Estimation is the process of finding the most probable value for a target parameter(s) based on a set of observable samples, which are correlated with the target parameter(s). Accuracy of the estimation can vary based on the quality of the observed samples. Quantifying the quality of estimation is one of the main subjects in estimation theory, and in most of the cases, it is an even harder problem than estimating the target parameter. A satellite based positioning system is one of the early systems that was introduced for global positioning, and for the same reason it is called Global Positioning System (GPS). In the GPS network, accuracy of estimation is also determined and reported to end users. The estimation error in the GPS network is presented in different ways. The error estimation is determined by considering the entire network, and it is called Delusion Of Precision (DOP) for horizontal and vertical error. The DOP value is an indicator of error, and it can be translated to error in meters as well.
Metro wide WLAN-based positioning systems have been explored by a couple of research labs, but none of them provided an expected error of position estimation. The most important research efforts in this area have been conducted by PlaceLab (www.placelab.com, a project sponsored by Microsoft and Intel), University of California San Diego ActiveCampus project (ActiveCampus—Sustaining Educational Communities through Mobile Technology, technical report #CS2002-0714), and the MIT campus wide location system.
There have been a number of commercial offerings of WLAN location systems targeted at indoor positioning. (See, e.g., Kavitha Muthukrishnan, Maria Lijding, Paul Havinga, Towards Smart Surroundings: Enabling Techniques and Technologies for Localization, Proceedings of the International Workshop on Location and Context-Awareness (LoCA 2005) at Pervasive 2005, May 2005, and Hazas, M., Scott, J., Krumm, J.: Location-Aware Computing Comes of Age, IEEE Computer, 37(2):95-97, February 2004 005, Pa005, Pages 350-362.) These systems are designed to address asset and people tracking within a controlled environment like a corporate campus, a hospital facility or a shipping yard. The classic example is having a system that can monitor the exact location of the crash cart within the hospital so that when there is a cardiac arrest the hospital staff doesn't waste time locating the device. The accuracy requirements for these use cases are very demanding, typically calling for 1-3 meter accuracy. These systems use a variety of techniques to fine tune their accuracy including conducting detailed site surveys of every square foot of the campus to measure radio signal propagation. They also require a constant network connection so that the access point and the client radio can exchange synchronization information similar to how A-GPS works. While these systems are becoming more reliable for indoor use cases, they are ineffective in any wide-area deployment. It is impossible to conduct the kind of detailed site survey required across an entire city and there is no way to rely on a constant communication channel with 802.11 access points across an entire metropolitan area to the extent required by these systems. Most importantly, outdoor radio propagation is fundamentally different than indoor radio propagation, rendering these indoor positioning techniques almost useless in a wide-area scenario.
There are millions of commercial and private WLANs deployed so far and this number is growing everyday. Thus, WLAN access points are used to estimate the location of WLAN-enabled mobile devices.